the advantages required, but which are entirely secured by Dr. H. Vogel's admirable invention described below.
The objects of this ingenious device is to enable the operator to judge of the proper time to expose a negative in the carbon printing process, and to determine the exposure when making negatives in the glass-room. Those who have experimented in carbon printing know the difficulty of determining the proper exposure, too short or too long a time destroying the print altogether. The use of this little instrument overcomes that. In making negatives on days when the light is variable, difficulty is often experienced in securing the proper time of exposure. This is also the case when making copies, views of interiors, and landscapes. A proper understanding and use of Dr. Vogel's photometer will be found of immense advantage in such cases. The length of exposure of any desired picture, in any desired weather, can at once be ascertained. Mr. Swan has adopted this photometer.
We shall now proceed to describe it.
It consists of a box A A, provided with a lid B B, as shown in the drawing. The lid consists of a frame, a,
by which a glass plate, b, is held; on the upper side of this glass plate is secured a series of thin strips of paper, which are arranged mathematically in layers; each lower
